OK, once again, a mad idea. I had to try this, upon reading an article in Spin Off magazine: unravel sweaters to get usable yarn.
So off to the Goodwill I went. The key is to look at the seams. If joined with a chain stitch, it means the garment pieces were shaped while knitting, and the yarn will pull continuously. If serged, then likely the garment was knit as fabric, then cut to size and sewn, and the yarn will only be as long as one row.
From a traditional men's lambswool vest, I got two balls of fine yarn (total 6 oz), costing $5.49
I ripped out the seams and took off the ribbing. And I will save the label until I use the yarn.
What made it easy was using my ball winder, pulling straight from the garment. This thin yarn did break, so I will knit or ply the yarn knowing that I will need to deal with a few knots or joins.